The present invention relates to control systems of the type having a plurality of remotely located process control units connected together through a communications link and, more particularly, to a control system in which each of the remote units is capable of testing the communication integrity of the communications link between it and other remotes in the system and automatically inserting a line-termination impedence when a degraded or interrupted channel in the communications link is detected.
Many system-type industrial installations, for example, those related to industrial process manufacturing and electrical power generation, employ a large number of physically distributed controlled-devices and associated sensors for effecting coordinated operation of the overall system. In the past, coordinated control of the various devices has been achieved by manually operated and various types of semi-automatic and automatic control systems including electromagnetic relay systems, hardwired solid-state logic systems, and various types of computer control systems. The computer systems have included central systems in which the various sensors and controlled devices are connected to a central computer; distributed control systems in which a remotely located computer is connected to each of the controlled devices and to one another; and hybrid combinations of the central and distributed systems. The successful functioning of the control system is vital to any industrial process, and, accordingly, distributed systems have generally been preferred over central systems because the failure of one of the remotely located control computers generally does not cause a system wide failure as in the case of the failure of the central computer in the central system. However, in the distributed systems, a communication link or buss interconnects each of the various remotes and deterioration or interruption of the communication link can effectively divide the system into inoperative portions. Such degradation or interruptions of the communication link can occur when the communication link physically passes through an area of high electrical noise that can induce errors into the data transmissions or where the communications link is actually severed. In the latter case, the open communications link causes an unbalanced line impedence which can adversely affect the data signal-current levels in the communications link. When the system transmits information in digital form at high and very high data rates, an interrupted transmission link can cause reflected signals which, in turn, can cause false triggering and garbled data transmission. In view of this, a control system must be able to detect a degradation or interruption of its communication link and be able to minimize the adverse affect on the communication capability of the remaining remotes in the system in order to maintain high reliability data transmission between remotes.